Leadless chip resistors are widely used in surface-mount technology electronic assemblies. Historically, the major users of chip resisters have been hybrid circuit manufacturers. Chip resistors have been used in place of thick film printed resistors for reasons such as ability to obtain extreme resistance values, unusual values, close tolerances, and to achieve lower overall cost. More recently, as the techniques of leadless component mounting have improved, the advantages of chip resistors have increased over printed resistors. These advantages are now recognized by conventional circuit assemblers on printed circuit boards.
Referring to FIG. 1, a leadless chip resistor 10 is formed on a ceramic body 12, having the shape of a rectangular prism or parallelepiped. On one face of the ceramic body, a thick film resistive element 14 is printed. Many types of different resistant inks are used in order to fabricate resistors with varying resistance values and varying tolerances. Two opposing ends of the resistive element are connected to an electrically-conductive terminations 16. The terminations employ numerous configurations such as wraparound or flip chip. In the wraparound style, the termination overlaps the resistive element and wraps around a second face of the ceramic and portions of the underside of the ceramic. Other styles of termination, such as flip chip terminations, simply have a pad of conductive material connected to the resistive element on the face of the ceramic body. Materials used for terminations are typically solder, gold, tin, lead, indium, silver, platinum, nickel, and combinations thereof. Most chip resistors used in wave soldering have a precious metal base coating covered by a plated nickel barrier layer and a top coating of tin/lead solder. The nickel barrier serves to prevent leaching of the precious metal base coat, thereby assuring a reliable electrical connection to the circuit board.
A protective glass passivation coating over the resistive element is sometimes used. This passivation eliminates the possibility of foreign materials, such as conductive epoxy, contaminating the resistive element and changing its value. Passivation also prevents solder from leaching the resistor body during soldering, which can cause minor resistance changes. The completed resistor is typically trimmed by laser or airabrasive techniques in order to achieve the desired resistance.
Prior art resistors (FIG. 1 and FIG. 3) all use a single element (14 and 34) on one side of the ceramic body. Resistive networks (FIG. 2) are made from a silicon body 22 and have numerous resistors 34 printed on one surface of the silicon and interconnected in various configurations. The silicon chip has many terminations 36 allowing the end user to custom select the resistance value using only a single component.
Although resistive networks have overcome some of the disadvantages of discrete chip resistors, namely the ability to have different resistive values in a single package, this result is achieved at the expense of a much larger package. Discrete, leadless chip resistors continue to suffer the disadvantage of requiring a unique package and part number for every resistive value and tolerance. This requires that many different types of chip resistors are used in assembling a single electronic assembly. Clearly, it would be advantageous to reduce the number of unique parts required. A resistive package employing more than one resistive value in a package and having a small size approaching that of a discrete chip resistor would be highly advantageous and eagerly sought by the surface mount technological industry.